A conventional extension rod unit 8 is shown in FIGS. 12 and 13, and generally includes an inner tube 81 and an outer tube 82 into which the inner tube 81 is retractably inserted, two support boards 83 are connected to two ends of the extension rod unit 8. A connection member 9 includes two lugs 91 extending from two ends thereof and the connection member 9 is fixedly mounted to the open end of the outer tube 82 by narrowing the distance between the two lugs 91.
The inner tube 81 includes multiple positioning holes 811 defined therein and a lever 94 is pivotably connected to the connection member 9. An engaging member 92 is located within the lever 94 and biased by a spring, the engaging member 92 includes teeth 93 which are removably engaged with the positioning holes 811 in the inner tube 81. The inner tube 81 is pushed upward by pivoting the lever 94 downward. A positioning member 95 is located in the connection member 9 and includes a pawl 951 which has an inclined surface, the pawl 951 is removably engaged with one of the positioning holes 811 to keep the inner tube 81 from sliding downward relative to the outer tube 82. A torsion spring 96 is used to keep positioning member 95 at the position such that the pawl 951 is always engaged with one of the positioning holes 811.
It is understood that the positioning holes 811 bear a significant stress when the pawl 951 is engaged therewith to prevent the inner tube 81 from collapsing downward, especially when a load is applied to the support board 83. However, the positioning holes 811 are not applied with heat treatment which may cause deformation of the positioning holes 811. Therefore, the inner tube 81 is weak and has a short term of use. Besides, the pawl 951 bears significant stress and is easily broken.
Furthermore, the movement of the inner tube 81 can only be adjusted by the distance between the positioning holes 811, so that when securing boxes for example, there will be small gaps between the boxes because the inner tube 81 cannot be micro-adjusted. The gaps cause impact between the boxes which may even drop during transportation.
The present invention intends to provide an extension rod unit wherein the inner tube can be micro-adjusted relative to the outer tube.